It's not quite as ubiquitous as you're implying:
a. if you're VFR squawking 1200, they don't know who you are, just that you're flying by, unless you've filed a flight plan they could correlate to your radar track
b. ADS-B provides identity info, but still isn't mandatory in Class E airspace under 10,000 feet (see
this graphic)
c. if both of those are off or not working, then ATC just gets a skin paint, which is next to useless except to say there is something (maybe a plane, maybe a big bird) there